How to make a clothing mesh follow an arm mesh?
To solve this:


Select your mesh and switch to edit mode
Select all using A
Hit P then choose By loose parts
Now there will be two meshes, the arm and the cloth. Select the cloth mesh and hit Alt+P then clear Parent
Select your cloth mesh then select the armature as well using Shift+ Right Click
Hit Ctrl+P and select with Automatic weighting


and this will be the result: